Flooding from monsoon rains covers a wide region of northeast Bangladesh near the India border in this Oct. 8, 2023, image showing data from the U.S.-French Surface Water and Ocean Topography (SWOT) satellite. Around that time, heavy monsoon rains affected various parts of Bangladesh, including the region depicted here, home to Sylhet, the country's fifth-largest city (population approximately 700,000). Sylhet and surrounding areas experienced substantial flooding.

The SWOT data, overlaid on a standard Google Earth image, shows the precise surface elevation of the floodwaters. The vast majority of the land area within the region imaged by SWOT is flooded.

In the main image at left, dark blue indicates waters just above sea level, while yellow represents 65 feet (20 meters) above sea level. The inset at top right zooms in on a section of the Kalni River, a tributary of one of Bangladesh's three major rivers, the Meghna. The main channel of the Kalni is about 650 feet (200 meters) wide. In the inset at top right, dark blue indicates water about 10 feet (3 meters) above sea level and yellow is used for waters about 16 feet (5 meters) above sea level.

The zoomed-in data can be used to determine the slope of the river, which tells hydrologists how fast water flows through it and off a landscape. The water surface elevations depicted are higher in the top right of the inset than they are in the bottom left. The data also shows the elevation of the river is generally greater than that of water in the floodplain around it, telling hydrologists that water is likely flowing from the river into the floodplain.

Each pixel in the image represents an area that is about 330 feet by 330 feet (100 meters by 100 meters). The image shows data from SWOT's Ka-band Radar Interferometer (KaRIn) instrument antennas, which don't cover a 12-mile-wide (20-kilometer-wide) strip right underneath the satellite. For that gap, a separate instrument collects water surface elevation data that isn't shown here.

Bangladesh is a low-lying, generally flat country that chronically suffers from two types of flooding. Monsoon rains cause rivers that flow into Bangladesh from neighboring countries, especially India, to flood. And flash flooding occurs during the pre-monsoon season and can damage agricultural crops if not forecast with sufficient warning.

While the use of satellite altimetry and precipitation in models has shown some promise for flood forecasting in Bangladesh, forecasting both types of flooding with sufficient lead time has remained challenging due to a lack of timely, high-spatial-resolution information on water elevation. SWOT is the first satellite to provide this information over entire regions, enabling improved forecasts and filling the data gap when places like Bangladesh become inundated.

The Bangladesh Water Development Board, which is in charge of national flood forecasting and management, is now exploring how to incorporate SWOT water elevation information into its flood inundation forecasting system.

Since shortly after launch in December 2022, SWOT has been measuring the height of nearly all water on Earth's surface, developing one of the most detailed, comprehensive views yet of the planet's oceans and freshwater lakes and rivers.

The mission science team makes measurements using the KaRIn instrument. With two antennas spread 33 feet (10 meters) apart on a boom, KaRIn produces a pair of data swaths as it circles the globe, bouncing radar pulses off water surfaces to collect surface-height information.

SWOT was jointly developed by NASA and the French space agency, CNES (Centre National d'Études Spatiales), with contributions from the Canadian Space Agency (CSA) and the UK Space Agency. NASA's Jet Propulsion Laboratory, which is managed for the agency by Caltech in Pasadena, California, leads the U.S. component of the project. For the flight system payload, NASA provided the KaRIn instrument, a GPS science receiver, a laser retroreflector, a two-beam microwave radiometer, and NASA instrument operations. CNES provided the Doppler Orbitography and Radioposition Integrated by Satellite (DORIS) system, the dual frequency Poseidon altimeter (developed by Thales Alenia Space), the KaRIn radio-frequency subsystem (together with Thales Alenia Space and with support from the UK Space Agency), the satellite platform, and ground operations. CSA provided the KaRIn high-power transmitter assembly.

To learn more about SWOT, visit: https://swot.jpl.nasa.gov/